Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 183

the strong antigenicity of Gn terminated glycans. As the glycans with Xyl and
Fuc groups, commonly found in plant glycoproteins, are highly immunogenic,
recombinant glycoproteins produced by plant cells may also cause pathological
concerns [222]. Artificial glycosylation sites have been introduced into small
peptides to improve their pharmacokinetic properties or to make them resistant
to proteases [223,224].Although an authentic human glycosylation profile could
be the final aim; it may be desirable to produce a modified glycoprotein with
defined glycosylation and predictable pharmacokinetic properties [192]. There
are four properties critical to the efficacy of therapeutic proteins depending on
glycosylation - biological activity,antigenicity,immunogenicity and circulatory
half-life. Since the roles of glycosylation in overall biological activities of glyco-
proteins have been discussed in the earlier sections, they are not separately
included here.

6.1
Antigenicity

Correct post-translational processing has a large impact on the efficacy and
antigenicity of recombinant protein [225].Frequently,the -linked sugars either

alone or in combination constitute antigenic determinants. Oligosaccharide
structures can serve as a basis for antibody recognition [226]. Glycoproteins
with nonnative carbohydrate structures may be antigenic. Many mammalians
circulating antibodies are targeted against specific oligosaccharide deter-
minants. Most humans have circulating antibodies against N -linked yeast
mannan chains, and about 1% of circulating human IgG is specific for the ter-
minal Gal

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1,3-Gal epitope generated by mouse C127 cells.There may be rapid
clearance due to antigen-antibody complex formation followed by phagocyto-
sis. Glycans may also contribute indirectly to glycoprotein antigenicity. The
interaction with antibodies is influenced by their bulky hydrophilic and often
highly charged glycan moieties. The glycans appear to adopt a flattened con-
figuration over the surface of the peptide moiety which would tend to maxi-
mize steric hindrance in binding to antibody [227]. Glycoproteins such as
human

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1-acid glycoprotein, which contains 5 glycan moieties essentially of
the tri- and tetra-antennary types, are completely enveloped by the glycans.
This could explain the resistance and the weak antigenicity of glycoproteins,
the glycans acting as protective shields. The umbrella configuration of glycans
is firmly maintained by ionic bonds between the electronegative charges of SA
residues and electropositive ones of basic amino acids.Removal of SA residues
makes the antennae free and mobile, abolishes the protective effect of glycans,
and the glycoprotein becomes more antigenic and more susceptible to degra-
dation. The protective role played by glycans towards the protein moieties
could explain the resistance of metastatic cancer cells,as their membrane glyco-
proteins are significantly enriched in tri- and tetra-antennary glycans [228].

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